The present invention relates to an optical submarine cable with a sheath and a core, which comprises optical fibres, wherein the fibres are enclosed in a metal tube which is disposed near the axis of the cable.
Optical cables have a core with optical fibres, in most cases glass fibres coated with one or more layers of polymer material for protection and identification. The cable core is enclosed in a sheath made from a polymer material, e. g. a polyolefine. For submarine cables, a common design comprises a central loose tube around the fibres. The tube is formed from a metal band and hermetically sealed by welding of the band edges. It is disposed on the axis of the cable and consists of steel to ensure a high protection of the fibres, especially from the hydrostatic pressure at the sea bed. Interstices in the core are generally filled with a hydrophobic jelly to avoid spreading of water in the lengthwise direction if the cable is damaged. In another common design of cables, a core element has slots wherein the fibres are disposed, often as fibre-ribbons comprising several fibres. A slotted core design has the advantage that a high number of fibres can be incorporated in the core.
Although steel tubes offer superior protection, tubes with a typical diameter of 3.7 mm can accommodate only up to approximately 45 to 50 fibres before the optical properties of the fibres deteriorate and the optical damping increases. If the diameter of the tube is increased while keeping a constant thickness of the wall, the mechanical stability gets worse. Therefore, the possible range of tube diameters for high pressure environments, e. g. for deep sea applications, is limited. Furthermore, it is difficult to distinguish a high number of loose fibres inside a tube, especially in view of the small fibre diameter. The use of tubes with different dimensions depending on the cable type can be disadvantageous because of the necessary time for change-over of the machinery and different welding parameters for the tubes.
It is therefore an object of the present invention to obviate these disadvantages and to develop a cable with a high- number of optical fibres protected by a metal tube. It is a further object, to provide a cable with a good flexibility and a low diameter. It is still another object of the invention to develop a cable wherein the properties of the metal tube can be independent of the number of fibres inside the cable.
According to the invention the cable comprises at least two metal tubes and the tubes are SZ-stranded.
A central idea of the invention is to incorporate more than one metal tube into the core of the cable. Because not more than one tube can be located on the axis of a cable, the tubes have to be stranded to avoid damage when the cable is bent. The core can comprise for example 2, 3, 4 or more tubes made from a high strength metal and stranded with each other. Alternatively or in addition, one or several layers of tubes can be stranded around a central tube, a central group of tubes or another central element. A central tube can also contain optical fibres.
The number of tubes is chosen according to the required number of optical fibres. As a result, the dimensions of the tubes can be chosen according to the necessary mechanical properties. The dimensions of the tubes are preferably, although not necessarily, identical so that all tubes can be manufactured with the same machinery without changes in the manufacturing parameters. The number of fibres in each tube should be limited to approximately 50 or below for a tube with a diameter of 3.7 mm to avoid a deterioration of the optical properties. The fibres are accommodated in the tubes with a defined excess length to avoid mechanical stresses due to temperature variations or forces on the cable. The outer dimensions of the cable vary only insignificantly with the number of tubes inside. The tubes offer the opportunity to accommodate a high number of safely protected fibres in a small volume. The accommodation of the fibres in more than one tube allows for an easy identification of the fibres and a small diameter of the tubes.
As submarine cables are manufactured with great length, SZ-stranding is necessary which allows an unlimited length of the stranded tubes. With a single sense of stranding, the length is limited by the size of the reels for the metal tubes which can be accommodated in the stranding machine. Suitable is an SZ-stranding with approximately five to seven lays before the sense of rotation is reversed. Depending on the properties and arrangement of the tubes in the core and of the required properties of the cable other numbers of lays are possible. The core is preferably provided with an outer wrapping for stabilisation of the stranded assembly, e.g. a polyester tape. Over the wrapping, a sheath is extruded which consists for example of polyethylene.
In a preferred embodiment, of the invention, the metal tube is a welded steel tube which is mechanically robust and offers a safe protection of the fibres.
Interstices in the core of the cable are preferably filled with a hydrophobic compound which avoids the spread of water inside the cable. The hydrophobic compound can also be applied on the outside surface of the wrapping to avoid spread of water between wrapping and sheath.
To ensure a round cross section of the cable with a well-defined diameter and to improve the stability, the core comprises filler elements. They are disposed in grooves and interstices between the tubes to result in an approximately round cross section of the core. Filler elements can also replace one or more tubes in the core. In this case, the diameter of the filler element is identical to the diameter of the tube which it replaces. A suitable material for the filler elements is polyethylene which has a limited flexibility and sufficiently low compressibility to withstand hydrostatic pressure.
In an advantageous embodiment of the invention, at least one filler element is optically distinguishable from another filler element. Preferably, one or several filler elements with different colours or markings are used. The filler elements are disposed in a defined relationship to the tubes and allow therefore an identification of the individual tubes.
Because steel tubes have a comparatively high specific electrical resistance, the core of a preferred cable comprises at least one bare conductor with a low electrical resistance like a copper tape or a copper wire. If the sheath of the cable is damaged, a current can be fed through the conductor for detection with a probe which is moved along the cable to locate the damaged section. It is possible that a copper conductor is a filler element.
Especially in shallow water with a depth up to several 100 meters, a cable is often subject to impact forces which may for example be caused by fishing gears. To avoid damage, the cable is preferably provided with an armouring which can consist of one or several layers of steel wires stranded around the sheath. The number of wires depends on the required protection, the acceptable weight and diameter of the cable. Optionally, a bedding layer consisting for example of impregnated paper is disposed between the sheath and the armouring. If the sheath has a sufficient thickness, a bedding can be omitted. To avoid corrosion, steel wires are galvanised with zinc and their interstices and their surfaces are covered with bitumen in a preferred cable.
For the protection of the armouring, an advantageous cable comprises an outer jacket. The jacket can consist of one or several layers of polymer yarn, especially polypropylene yarn, wherein at least the innermost layer adjacent to the armouring is flooded with a watertight material such as bitumen. Alternatively, the jacket is a compact sheath consisting of a polymer material like polypropylene, polyurethane or polyethylene.